Reaction and Transport Coupling in Multi-Functional Catalysts for Selective Oxidation of Ammonia to Nitrogen

A multi-functional catalytic reactor (popularly known as “Ammonia Slip Catalyst (ASC)” or “AMOx”) comprising Pt/Al2O3 and Cu or Fe-exchanged zeolite is utilized for selective oxidization of NH3 to N2 in the diesel engine emission control system. Pt catalyzes the oxidation of NH3 to four products (N2, N2O, NO, NO2) while the metal-exchanged zeolite concurrently catalyzes the selective catalytic reduction of NO/NO2 with NH3, directing the product mixture towards N2. The state-of-the-art ASC has a dual-layer washcoat architecture with a bottom layer of Pt/Al2O3 and a top layer of Cu/SSZ-13. As emission control catalysts experience degradation under the severe operating conditions which can negatively impact the product distribution and conversion. The first part of this dissertation study elucidates the effect of hydrothermal aging on the performance of a Pt/γ-Al2O3, Cu/SSZ-13 and dual-layer washcoated monolith used for oxidation of ammonia. The catalysts was subjected to a feed stream containing H2O at 550 oC for over 250 hours and performance was measured at discrete intervals as it was progressively aged.
In the next part of this study, the enhanced mass transport phenomenon coupled with multi-functional catalytic reactions in ASC is investigated. Earlier works have shown that pore diffusion can be limiting in ASC. we used sacrificial agents (polymer, yeast) to increase the porosity of washcoat for ASC. The sacrificial agents are removed by oxidation generating pores to increase the washcoat macroporosity. Steady state reaction experiments showed a noted increase in NH3 conversion while testing NH3 oxidation the dual-layer ASC with negligible change in N2 selectivity. Modeling and analysis of ASC catalysts is presented using 1 + 1 dimensional reactor model containing tuned multi-step kinetic formulations to simulate ASC performance for understanding and optimization. We also describe a systematic study to identify the ASC architecture and composition that optimize the trade-off between conversion and selectivity. The in-house synthesized ASC samples span the single layer Pt/Al2O3, conventional dual-layer Pt/Al2O3 + Cu/SSZ-13, uniform single layer of mixed Pt/Al2O3 + Cu/SSZ-13, and a hybrid design comprising a bottom layer of mixed Pt/Al2O3 + Cu/SSZ-13 and a thin top layer of Cu/SSZ-13.